This invention relates to ophthalmological surgery techniques which employ an ultraviolet laser used to provide photodecomposition of the surface of the cornea in order to correct vision defects.
Ultraviolet laser based systems and methods are known for enabling ophthalmological surgery on the surface of the cornea in order to correct vision defects by the technique known as ablative photodecomposition. In such systems and methods, the irradiated flux density and exposure time of the cornea to the ultraviolet laser radiation are so controlled as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea, all in order to correct an optical defect. Such systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for "METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for "METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER SURGERY METHOD AND APPARATUS"; and U.S. patent application Ser. No. 081,986 filed Aug. 5, 1987 for "PHOTOREFRACTIVE KERATECTOMY".
In the above-cited U.S. Pat. No. 4,665,913 several different techniques are described which are designed to effect corrections for specific types of optical errors in the eye. For example, a myopic condition, which is typically caused by excessive curvature in the anterior surface of the cornea, is corrected by laser sculpting the corneal surface to flatten the curvature. In addition, an astigmatic condition, which is typically caused by a cylindrical component of curvature departing from the otherwise generally spherical curvature of the surface of the cornea, is corrected by effecting cylindrical ablation about the axis of cylindrical curvature of the eye. Other optical errors can be corrected in a similar fashion.
The technique for providing the flattening of the corneal curvature for myopia error correction involves selectively varying the area of the cornea exposed to the laser beam radiation to produce an essentially spherical surface profile of reduced curvature. This selective variation of the irradiated area may be accomplished in a variety of ways. U.S. Pat. No. 4,732,148 cited above discloses the technique of providing a movable opaque element having apertures of various diameters and passing the laser beam through different ones of the apertures in a programmed fashion, starting either with a smallest diameter aperture and progressively increasing the surface area of exposure using apertures of wider diameters, or using the reverse process. Another technique for accomplishing varying areal exposure employs a variable diameter iris for controlling the area of the cornea exposed to the laser beam. Still another technique for providing the flattening of the corneal curvature for myopia error correction involves the use of a laser beam attenuator which varies the energy distribution of the laser beam to sculpt the surface of the cornea in conformance with the varied energy distribution. The attenuator typically includes a positive lens-shaped portion with a laser energy absorbing material and end caps having planar outer surfaces and the same refractive index as the positive portion, which prevents refraction of the laser beam upon passing through the attenuator. This technique is disclosed in U.S. Pat. No. 4,838,266, issued Jun. 13, 1989 for "LENS SHAPING DEVICE USING A LASER ATTENUATOR", the disclosure of which is hereby incorporated by reference. The astigmatic cylinder correction is typically performed by providing a pair of movable blades which intercept the laser beam and permit only a rectangular area of the cornea to be exposed to the beam through the width of the slit formed by the confronting edges of the blades, and by controlling the width of the slit in a predetermined manner so that a rectangular area of the cornea of either increasing or decreasing width is exposed to the laser beam. The '466 U.S. patent noted above discloses such a variable width slit mechanism.
In practice, the laser sculpturing ophthalmological surgical system is typically provided with delivery system optics which include both the variable diameter beam shaping element and the variable width slit mechanism in order to afford both myopia and astigmatism corrections. In some patients, there are both myopia and astigmatism defects in the same eye, requiring correction of both errors in order to improve vision. In the past, such compound errors have been corrected in systems having a variable diameter element and a variable width slit mechanism in a sequential fashion, with the astigmatic correction typically being performed first with the slit mechanism, followed by the correction for myopia using the variable diameter element. This has the disadvantage that the length of the operation is maximized, which increases the time that the patient's eye must be completely immobilized. This increases the physical strain and stress on the patient.
In addition, the cylindrical ablations required to correct astigmatic errors normally result in sharp transitions in the cornea at the extreme ends of the sculpted area. It has been observed that the eye responds to such sharp transitions by promoting growth of the epithelium and the stroma to smooth out sharp edges in the surface of the cornea. This has an adverse optical effect, sometimes termed the "hyperopic shift", which causes vision regression and thus reduces the effectiveness of the laser sculpting technique. In addition, such sharp transitions have the potential to induce changes in corneal curvature, such as flattening along the cylindrical axis of ablation. In the past, attempts have been made to reduce the hyperopic shift by laser sculpting smoothing transition zones. This has been accomplished by manipulating the diameter of a circular aperture at the ends of the slit to form sigmoidal or "s" shaped transition zones. However, therapeutic patients undergoing large area ablations questionable since many of such patents still exhibit hyperopic shifts.